1. Field of Invention
This invention generally relates to processing compressed digital images. More particularly, this invention relates to methods and systems which reduce the boundary ringing artifacts occurring in hybrid compression.
2. Description of Related Art
Data compression is required in data handling processes, where too much data is present for practical applications using the data. Commonly, compression is used in communication links to reduce the transmission time or required bandwidth. Similarly, compression is preferred in image storage systems, including digital printers and copiers, where "pages" of a document to be printed are stored temporarily in precollation memory. The amount of media space on which the image data is stored can be substantially reduced with compression. Generally speaking, scanned images, i.e., electronic representations of hard copy documents, are often large, and thus make desirable candidates for compression.
A number of different compression techniques exist, and many of these are proprietary to individual users. The image compression standard disseminated by the Joint Photographic Experts Group (JPEG) committee is a compression technique which reduces data redundancies based on pixel-to-pixel correlations. Generally, picture regions of an image do not change very much on a pixel-to-pixel basis and therefore have what is known as "natural spatial correlation." In natural scenes, correlation is generalized, but not exact. Noise makes each pixel somewhat different from its neighbors.
Lempel-Ziv-Welsh (LZW) encoding, a popular run-length coding standard developed by J. Ziv and A. Lempel, and later refined by T. Welsh, is a technique for compressing data that takes advantage of repetition of strings in the data. Generally, raster data contains a high number of repetition. LZW compression is easy to implement, operates at high speed and results in high compression ratios without loss of data.